A DFTS-OFDM based single carrier transmitting method is used in an uplink of LTE mobile communication. The reason for using a single carrier modulating method in the uplink is that a peak-to-average power ratio (PAPR) is lower in the single carrier transmitting method than in a multi-carrier transmission method such as OFDM. As the PAPR of a transmitted signal is smaller and smaller, average transmission power can be increased with respect to a given power amplifier. Accordingly, single carrier transmission brings about higher power amplifier efficiency and this means an increase in coverage and a decrease in consumption of terminal power. Simultaneously, the single carrier transmission which occurs by selective frequency fading brings about the higher power amplifier efficiency and this means, that is, the increase in coverage and the decrease in consumption of the terminal power.
In spite of the same carrier transmitting method, an LTE uplink is based on orthogonal transmission in which uplink transmission is orthogonally split in a time domain and/or a frequency domain in contrast with a WCDMA/HSPA uplink which is non-orthogonal transmission. Since the orthogonal transmission can avoid intra-cell interference, the orthogonal transmission is more advantageous than the non-orthogonal transmission. However, allocating a very large instantaneous bandwidth resource for transmission from a single user equipment is not an efficient strategy under a situation in which a data rate is primarily limited by transmission power of the user equipment rather than a bandwidth. Under this situation, the corresponding user equipment is allocated with only a part of an entire bandwidth and simultaneously, other user equipments can transmit the signal by using a remaining spectrum part in an entire available bandwidth. Accordingly, since the LTE uplink has multiple connection elements in the frequency domain, the LTE uplink transmission method is called SC-FDMA.
A basic structure of DFTS-OFDM transmission in which M discrete Fourier transforms (DFTs) are applied to blocks of M modulated symbols is summarized. A DFT output is generally mapped to a selective input into an OFDM modulator implemented by inverse FFT (IFFT). The DFT output is generally mapped to the selective input into the OFDM modulator implemented by the inverse FFT (IFFT). The size of the DFT decides an instantaneous bandwidth of the transmitted signal and the mapping of the DFT output into the input of the OFDM modulator decides the position of the transmitted signal in an entire uplink cell bandwidth. Thereafter, a cyclic prefix (CP) is inserted into each DFT block. The LTE uplink transmission is limited to localized transmission, and as a result, the output of the DFT is continuously mapped to a consecutive input of the OFDM modulator.
In the LTE, as intermediate compromise between two purposes of easiness of implementation and flexibility of resource allocation, a DFT symbol size is limited to a value acquired by combining and multiplying 2, 3, and 5. For example, the DFT having sizes of 60, 72, and 96 is permitted, but DFT having a size of 84 is not permitted. In this method, the DFT is implemented by combining radix-2, radix-3, and radix-5 FFT processing having comparatively low complexity.
Basic parameters of the LTE uplink transmission method are decided to maximally coincide with parameters of an OFDM based LTE downlink. In general, an uplink subcarrier interval is 15 kHz in the LTE uplink transmission method and a resource block constituted by 12 subcarriers is defined even in the LTE uplink. Similarly to the downlink, even in the uplink, an LTE physical layer specification enables the number of various uplink resource blocks which is minimum 6 to maximum 110 to be used to achieve high flexibility in terms of the entire cell bandwidth. The LTE uplink is very similar to the downlink even in terms of a time domain structure. Each 1-ms uplink subframe is constituted by two slots having a length of 0.5 ms. Each slot is constituted by several DFTS-OFDM symbols including the CP. Further, similarly to the downlink, the lengths of two CPs of a general CP and an extended CP are defined even in the uplink.
Meanwhile, in a satellite system, high-output signal transmission is required due to a small link margin. 180 kHz which is a basic uplink transport block size in the LTE has a disadvantage in that power allocated in each subcarrier of a basic transport block cannot provide transmission power which can satisfy the link margin of the satellite system when a lower maximum transmission power level of a handheld type terminal is considered. Further, a basic transport block size of 180 kHz is provided in a 1-ms subframe length which is a minimum unit of the resource allocation, and in the case of Antenna Port 1, since 160 resource elements (REs) are provided as the minimum resource allocation unit, a data rate of 160 symbol/s cannot be supported and a low-rate data service of 16 kbps or less cannot be supported on an LTE radio interface standard specification. This imposes a limitation to the number of simultaneous access users.